As the markets for portable electronic devices such as mobile phones, camcorders and notebook computers have been expanded and diversified, the demand for a rechargeable secondary battery for power supply has also been increasing. Due to miniaturization, weight-lightening, high performance, and multiple functions of portable electronic devices, a secondary battery used as a power supply source has been required to be continuously improved in energy storage density. Therefore, years of research to meet such a requirement have yielded the current lithium ion secondary battery that adopts a carbon anode into and from which lithium can be reversibly inserted and extracted and a cathode material into and from which lithium can be reversibly inserted and extracted.
The lithium ion secondary battery has a higher energy density per unit weight and increased charge and discharge lifetime as compared with the existing aqueous solution type secondary batteries such as nickel-cadmium and nickel-hydrogen secondary batteries, and thus has been recently substituted as a new energy source for portable electronic devices for the existing batteries. However, along with the rapid development and diversification of portable electronic devices, a demand for batteries with a higher energy density and various sizes is rapidly increasing. Thus, the current lithium ion secondary battery does not meet such a demand.
In particular, the rapid trend to manufacture slim and small electronic devices increases demands for slim lithium ion secondary batteries having a small thickness. However, adoption of the existing manufacturing methods for cylindrical or prismatic lithium ion secondary batteries may cause drastic lowering of energy density per volume in manufacturing slim batteries. Therefore, it is considered that the development of a slim lithium ion secondary battery with a higher energy density per unit volume is essential in achieving miniaturization, weight-lightening, and slimming of various portable electronic devices.
In order to increase an energy density per unit volume of a secondary battery, it is necessary to accurately laminate or stack a cathode plate, a separator film, and an anode plate. That is, it is necessary to laminate or stack an electrode plate while an area of the anode plate is maintained to be greater than a coating area of a cathode active material on the cathode plate. If an active material coating portion on the cathode plate is not accurately positioned within the anode plate while the cathode plate and the anode plate are laminated or stacked, the performance of the battery may deteriorate.
In Korean Patent No. 10-0337707, the applicant of the present invention suggested a pocketing electrode plate in which any one of a cathode electrode plate or an anode electrode plate is wrapped with a separator film and an insulating polymer film to accurately laminate or stack the cathode electrode plate and the anode electrode plate. In order to form the pocketing electrode plate, it is necessary to form multiple punched spaces in the insulating polymer film for accommodating an electrode plate. In this process, the insulating polymer film is wasted. That is, the insulating polymer film corresponding to the punched spaces is not used but wasted.